1. Field of Invention
This invention relates generally to linear frequency modulated (LFM) laser radar systems and, more particularly, to a frequency modulation control system for LFM laser radar which utilizes a closed loop feedback signal to maintain accurate linearity in the frequency modulation of the transmitted signal.
2. Discussion
Laser radar systems which employ an intensely focused beam of light to detect the presence, position and motion of objects, have been used in numerous applications, especially in the radar communications and measurement fields. Militarily, these systems have been implemented in conjunction with cruise missile and tactical fighter technology wherein laser radar has provided functions such as obstacle avoidance and terrain following. These laser radar systems enable sophisticated target homing capabilities for accurately guiding a missile or plane toward a target by using a distinguishing feature of that target.
Linear frequency modulated (LFM) "chirped" laser radar has proven to be particularly useful in these applications. A "chirped" laser radar system typically includes a continuous wave (CW) transmitter which emits laser light at a pre-selected center frequency. This emitted light is frequency modulated into linear "chirps" by passing it through an electro-optical device disposed within the cavity of the transmitter. A voltage versus time signal applied to this electro-optic device, typically an electro-optical crystal, creates a laser beam signal which varies in frequency with respect to time.
The exact shape of the modulated waveform can be varied in order to optimize trade-offs in efficiency, complexity and performance. A bi-directional type of waveform, as shown in FIG. 1, is often used to minimize range/doppler ambiguity in the system as well as to maximize the ambiguous range. To create this waveform, the radio frequency of each transmitted pulse is both increased and decreased at a constant rate within the length of the pulse. The frequency variation is preferably linear and the frequency versus time characteristic of the signal is often a trapezoid pattern as shown by the solid line in FIG. 1. Each chirp in this type of waveform includes an "up chirp" component wherein frequency is increasing and a "down chirp" component wherein frequency decreases.
A transmitted chirped signal is directed toward a target and then reflected back therefrom, creating a return signal or "echo" associated with the target. The time taken by the transmitted signal to reach the target and then return causes the return signal to be displaced in time with respect to the transmitted signal. This is shown graphically in FIG. 1 wherein the solid line represents the transmitted signal Tx and the dashed line shows a corresponding return signal Rx. The instantaneous frequency difference between these signals can be used to "demodulate" the return signal in order to obtain information about the target.
To obtain both long range detection capabilities as well as fine resolution, the laser radar system typically utilizes a relatively long coded pulse as the transmitted signal and then takes advantage of pulse compression of the return signal to obtain a narrower pulse. This enables achievement of the increased detection ability of a long pulse radar system while also retaining the range resolution capability of a narrow pulse system. Transmission of long pulses also permits a more efficient use of the average power capability of the radar without generating high peak power signals.
Pulse compression allows the transmission of modulated pulses of sufficient width to provide the average power necessary to illuminate targets, at a reasonable level of peak power. The received echoes are compressed by decoding their modulation to obtain the range accuracy and resolution equivalent to that of a short pulse. This is accomplished by increasing the transmitted signal bandwidth by modulating the frequency of the carrier within the transmitted pulse.
Methods of pulse compression are essentially matched filtering schemes in which the transmitted pulses are coded and the received pulses are passed through a filter whose time-frequency characteristic is the conjugate (opposite) of the coding. This function is usually performed within the signal processor used to process the received echo. The matched filter, usually a Sound Acoustic Wave (SAW) matched filter, introduces into the signal a time lag that is inversely proportional to frequency. The SAW filter, within its operating range, has a delay versus frequency characteristic which is matched to the frequency versus time characteristic of the return (and, therefore, also the transmitted) signal.
As illustrated in FIG. 2, a filter for compressing an up chirp has a signal transit time which decreases linearly with increasing frequency, at exactly the same rate as the frequency of the echo increases. The trailing portions of an up chirp echo, being of a progressively higher frequency, take less time to pass through the compression filter than the leading portions, thereby causing successive portions to bunch together and become compressed. When a pulse has been compressed by the filter, its amplitude is much greater and its width much less than when it entered. While the output echo may be only a fraction of the width of the received echo, it can have many times the peak power.
In order to ensure an accurate compression process, the frequency variations of the transmitted signal must be properly matched to the time-frequency characteristic of the filter, each preferably being precisely linear and having a constant slope. Non-linearities introduced into the transmitted signal may cause large sidelobes on either side of the compressed pulse which can make the system unreliable as an obscured second target detection sensor. Also, the compressed pulse width may become significantly larger than the capability of the SAW matched filter. This results in a decreased ability to measure range and reduces the ability to resolve multiple targets. Finally, non-linearity also causes reduced efficiency of integration which results in a lower signal to noise ratio.
A graphic illustration of such a non-linearity occurring in the up chirp component of the transmitted beam Tx(up) is shown in FIG. 1. The same type of non-linearity could also occur in an analogous fashion to the down chirp component TX(dn) but this is not shown in FIG. 1 for purposes of clarity. Such a non-linearity occurs where, for any given time segment, the frequency change in the transmitted signal is not constantly linear. This can also be thought of as an error in slope or a deviation from the desired linear slope of the ramped up or down chirp. If allowed to occur, this non-linearity is reflected back in the return signal Rx thereby causing the above-mentioned problems in the pulse compression process.
Constant increases in demand for longer operating ranges in LFM radar systems have been dramatically increasing the linearity requirements of the LFM modulated waveform. However, perfect linearity in the transmitted signal is difficult to achieve. This is mainly due to inherent limitations in the electro-optical crystal and its high voltage driver. Such high linearity requirements have proven impossible to achieve with conventional open loop laser radar systems. In these conventional open loop systems, a two percent linear slope error allows the system to be functional but not without problems. While various closed loop frequency control methods have been provided for other types of radars to help increase transmitted signal linearity in these systems, none have as yet been effectively applied to laser based radar systems, in particular LFM chirped laser radar.
There is therefore a need for a chirped LFM laser radar system which provides sufficient linearity to produce longer operating ranges than achievable with current systems.